Thermionic electron emission device having low heat loss



United States Patent [56] References Cited UNITED STATES PATENTS [72] Inventor Luther Earl Blankenship I-lillsboro,0reg.

HXXX 777 3333 1333 NNW 111 333 n a m u u k n "u r a u N P m m S mm & m mmnm m .m Cu 0 u u .00 WW WL mmmo mm 1 h w arm s ee-lu CR. KNKS m .m h 8667 n 4566 rm 9999 He 1111 n l/l/ .n k 6337 m c lau mxB 6888 117 1 ,J Um 3503 aim 4743 m w 2 nsn 2333 PAA n W .O .m m 8 c wmmow m. mm 2 9 2.0" 0 7 2 m i ..v fl mmmm 7% TBI 0. de N mm n 1. g p ell Phms AFPA 11.1.1. i253 2247 .lrlrilt ABSTRACT: An indirectly heated cathode of small size and low power dissipation is described including a cathode member mounted on a plurality of hollow tubular supports of low' heat conductivity material to reduce heat conduction loss. ln

[ TBERMIONIC ELECTRON EMISSION DEVICE addition, a heat shield of ceramic material is provided around FIAV N L HEAT Loss the cathode member and its heater filament to reduce heat 11 claims 2 Drawing 8 radiation loss. As a result the cathode requires only 0.4 watt [52] U513 of heater input power to heat the cathode to approximately 800 C. and cause an electron discharge current of approximately 10 milliamperes to be emitted from an emitting sur- [51] Int.

, ratio of emitting area per input power is very high for the present cathode at .02 square inch per watt.

PATENZEDBEE QIQYG LUTHER EARL BLANKENSHIP lA/VE/VTOR BUG/(HORN, BLORE, KLAROU/ST 8 SPAR/(MAN 47' TOR/VEYS ,THERMIONIC ELECTRON EMISSION DEVICE HAVING LOW HEAT LOSS I BACKGROUND or THE INVENTION The subject of the present invention relates generally to thennionic electron emission devices and in particular to indirectly heated cathodes having low heatloss and requiring littleinput power. Conduction heat loss is greatly reduced by mounting the cathode member on a plurality of hollow tubular 1 cathode'supportsof low heat conductivity material. Radiation heat loss is reduced by providing a heat shield of ceramic material around the cathode and itsheater filament. The cathode structure 'of the present invention is especially useful in small electron tubes of lowpower dissipation. For example, the present cathode structure can .be employed in a cathode ray tube used as part of a portable television receiver.

- However, the eathod'estructur'e can also be employed in. other devices such as the cathode ray tube of an oscilloscope or a bistable storage v tubein, which case the flood gun of such storage tubecan be formed by the present cathode structure.

Previous indirectly heated cathodes have employed cathode support members of low conductivity material, as'shown in u.s. Pat. No. 3,333,138 of c. s. Szegho. However the input power for such cathode" is still excessive, being twice the.

amount of power required for the present cathode and having an emitting surface area of less than one half that of the present cathode. As a result, the ratio ofemittingsurface area to input power for the prior art cathode is approximately .005 square inch per watt, while such ratio is much greater for the cathode of the present invention, being approximately .02

square inch per watt at the same temperature.

ln -addition,..the cathode structure of-the present invention has the further advantage of being much stronger and less expensive to manufacture, due to the use. of; straight, tubular cathode supports rather than'the complexgV -shaped supports of U.S. Pat. No. 3,333,l38. Thus, due to their length and flat strip construction, the cathode supports ofthat patent would act somewhat like leaf springs to cause vibrational movement of the cathode with many resulting problems including possible damage to the heating element.

It is therefore one object of the present invention to provide an improved thermionic electron emiss'ion'device of small size an improved indirectly heated cathode structure for a cathode ray tube having a high ratio of electron emissive area to power input. I b 1 BRIEF DESCRIPTION OF DRAWINGS '3 Other objects and advantages of the present invention will be apparent from the following detailed description of a I I preferred embodiment thereof and from the attached drawingsof which: v

.FIG. '1 is a perspective view of the cathode structure of the present invention with parts brokenaway for greater clarity; 65

and

DETAILED DESCRIPTION or PREFERRED EMBODIMENT HO. 2 is a verticalse'ction view takenalong the line 2-2 of I cal cap of approximately 0.l inch diameter whose closed end is coated on its outer surface in a conventional manner with a layer 12 of electron emissive material consisting of, for example, the alkali earth oxides such as oxides of barium and strontium, or oxides ofbarium, strontium and calcium. Three hollow tubular cathode support members 1'4 are attached to the sides of the cathode member 10 adjacent its open end in any suitable manner such as by spot-welding. The tubular supports are of a low thermal conductivity metal, such as a 42 percent nickel steel or an alloy of about 29 percent nickel, 17 percent cobalt and percent iron called KOVAR," which have a conductivity of approximately .039 calories per second per centimeter per degree centigrade at room temperature and increase to a conductivity of about .05 in the operating temperature range of approximately 800 C. The tubular supports 14 have an outer diameter of .01 inch and an inner diameterof .008 inch, with a wall thickness of .001 inch. Thus the crosssectional area of the tubular supportsisextremely small and this, together with the low thermal conductivity material greatly reduces conduction heat loss from the cathode member 10 to such supports. However, such tubularsupports are quite strong and rigid so that they prevent any appreciable vibrational movement of the cathode.

In order to reduce radiation heat loss, a heat shield of ceramic material including a base member-l6 and a sleeve member 18 isprovided around the cathode member 10. The tubular cathode supports 14 extend through holes in the base member 16 and are attached by an annular connector ring 20 to the base member. The connector ring 20 is of approximately the same0.l inch diameter as the cathode member 10 and is attached by brazing its inner surface to an annular shoulder22 extending centrally from the bottom of the base member 16. The lower ends of the tubular cathode supports 14 are attached to the outersurface of the connector ring 20 by spot welding or other suitable technique.

A coiled filament heater 24 of refractory metal, such as an alloy containing approximately 3 percentrhenium and 97 per cent tungsten, is provided within the'cathode member 10 and has its opposite ends attached by spot welding to a pair of heater support rods 26. The heater support rods 26 extend through holes in the base member 16 and the lower ends of such rods are secured to the base member by brazing to the lower surface thereof. The base member 16 and sleeve member 18 are made of a suitableceramic material, such as alumina, which is of a white color torefiect the heat radiated from the cathode member 10 and heater 24. The ceramic sleeve 18 also provides a support spacer of low thermal expan- As shown in FIGS. 1 and 2, the thermionic electron emis "INCO 220. The cathode member is sion for a first anode 28 and a second'anode or field shaping electrode 30, so that the interelectrode spacing between such anodes andbetween'the cathode and the anodes does not change appreciably with heat. Also the coefficient of expansion of the material used in the tubular cathode supports closely matches that of the ceramicsleeve so there is little relative movement between the cathode and anodes of other anode. The second 30 is in the form of an annular ring having a downwardly projecting conical portion 38 bounding a central aperture 40. The second anode is attached by brazing its outer periphery to a shoulder 42 provided on the inner surface of the ceramic sleeve 18. The aperture 40 in the secondanode 30 is of slightly larger diameter thanthe aperture 34 in the first anode 28 and such second anode actsas a field correction electrode. The apertures of the anodes are in coaxial alignment with the emitting surface 12 of the cathode. As a result the present device may produce a wide beam of electrons which uniformly bombard the storage target of a bistable storage tube when such cathode is used as the flood gun of such storage tube. The second anode member 28 is approximately .3 inches diameter and .3 inch long, so that the thermionic electron emission device of the present invention is of extremely small size. In addition, the present device provides the same emitting surface area, which is about 0.1 inch diameter, as a conventional 2 watt cathode and yet requires only 0.4 watt of input power.

A lead conductor strip 44 of nickel or other suitable metal is spot welded to the periphery of the second anode 30 and passes through a notch in the outer edge of the ceramic base member 16 to enable a suitable electrical potential to be applied to such a second anode. All of the electrodes including the cathode l and heater 24 as well as anodes 28 and 30 may be attached to pins 46 provided on a tube base member 48 of plastic material and extending through the end of the glass envelope 50 of a cathode ray tube or other suitable electron discharge device containing the cathode structure of the present invention.

When a supply voltage of 6.3 volts is applied to the heater 24', a heater current of approximately 65 milliamperes flows and causes the cathode to heat to an operating temperature of approximately 800 C. with an input power of 0.4 watts. This produces cathode emission current of approximately milliamperes with a cathode emitting surface of 0. l inch diameter. Since the total emitting surface area is approximately .008 square inch, the ratio of emitting surface to input power is approximately .02 square inch per watt which is much higher than previous cathodes. This high ratio is due to the low heat loss device.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the abovedescribed preferred embodiment of the present invention without departing from the spirit of the invention. For example, a metal sleeve may be employed as the heat shield. Therefore the scope of the present invention should only be determined by the following claims.

I claim:

1. A thermionic electron emission device, comprising:

a cathode member having an electron emitting surface;

a heater element;

support means for supporting the cathode member adjacent the heater element so that said cathode member can be heated by said heater element sufficiently to cause electrons to be emitted from said emitting surface; and

said support means including a mounting member and a plurality of hollow tubular cathode supports attached between said mounting member and said cathode member for reducing conduction heat loss and enabling said cathode member to be positioned close to the mounting member.

2. A device in accordance with claim 1 which also includes heat shield means of ceramic material surrounding the cathode member and the heater element for reducing radiation heat loss, said cathode member being separated from said heater element by free space.

3. A device in accordance with claim 1 in which the tubular cathode supports are of low heat conductivity material and are the only supports attached to the cathode member.

4. A device in accordance withclaim 2 in which the heat shield means includes a ceramic base portion providing the mounting member on which the tubular'cathode supports are mounted.

5. A device in accordance with claim 4 in which the cathode supports extend through the base portion and are connected together by a connector member secured to said cathode supports and attached to the base portion on the opposite side of said base portion from the cathode.

6. A device in accordance with claim 5 in which the cathode member is a cylindrical cap and the connector member is in the form of a ring of substantially the same diameter as said A device in accordance with claim 6 in which the emitting surface is provided as a coating of electron emissive material on the outside of the closed end portion of said cap and the heater element is supported on support rods extending through the base portion of the heat shield to position the heater element within the cap adjacent the inside of the cap end portion.

8. A device in accordance with claim 7 in which the heat shield means includes an annular sleeve portion on which is mounted a first anode surrounding said sleeve portion and a second anode positioned between said first anode and the cathode emitting surface so that apertures in said anodes are aligned with said emitting surface.

9. A device in accordance with claim 3 which also includes at least one anode member supported on the heat shield means so that an aperture in said anode member is in alignment with the emitting surface.

10. The electron emission device of claim 1 in which the support means includes means for mounting said device within the envelope of an electron tube.

11. A cathode assembly for an electron tube, comprising:

a cathode member having an electron-emitting area;

heater means disposed adjacent said electron-emitting area;

mounting means spaced from said cathode member; and

a plurality of hollow support means of low heat conductivity material for reducing conduction heat loss of said cathode member extending between said cathode member and said mounting means and for supporting said cathode member spaced from said heater means. 

